Flat panel display devices generally provide relatively good display quality, efficient space utilization, low power consumption, and absence of radiation. Thin film transistor (TFT) liquid crystal display (LCD) devices are a popular type of flat panel display device.
FIG. 1 depicts a circuit schematic diagram of a portion of a TFT LCD device. The LCD device has an array of thin film transistors TFT10A, TFT10B, TFT10C, and so forth, for respective pixels P10A, P10B, P10C, and so forth. The thin film transistors TFT10A, TFT10B, TFT10C and pixels P10A, P10B, P10C are arranged in a row of the LCD device, where all TFTs in this row are driven by the same scan line S10. There are multiple rows of TFTs an pixels along respective scan lines. When a scan line S10 is driven to a sufficient activation voltage (by a scan line driving circuit 12), the TFTs TFT10A, TFT10B, TFT10C, and so forth, that are connected to the scan line S10, will be turned on, so that the data (voltage level) carried on each of the data lines D10 can be written into the respective pixels P10A, P10B, P10C, and so forth. The data lines D10 are driven by a data line driving circuit 14. Upon completion of the pixel writing, the TFTs TFT10A, TFT10B, TFT10C, and so forth, are turned off, and the voltage level of the pixel electrodes in each of the pixels P10A, P10B, P10C, and so forth, are maintained by a respective liquid crystal capacitor and the pixel storage capacitor.
However, when the TFTs are turned off, the voltage level of the pixel electrodes in corresponding pixels can change due to coupling effects from voltage changes on the scan line. The voltage change of a pixel electrode due to coupling effect is referred to as feed-through voltage (VFD). The feed-through voltage can generally be expressed as:VFD=[CGD/(CLC+CST+CGD)]×ΔVG,  (Eq. 1)where CLC in Eq. 1 is the liquid crystal capacitor, CST is the pixel storage capacitor, CGD is the capacitor between the gate electrode and the drain electrode of the TFT, and ΔVG is the voltage difference between the activation voltage of the TFT and the turn-off (or deactivation) voltage of the TFT, on a scan line. Note that other capacitances in each pixel may also contribute to VFD, but such other capacitances are typically of relatively low values.
In an LCD device, the rotation angle of liquid crystal molecules is changed by controlling the size of the electric field applied to the liquid crystal molecules to vary the displayed gray scale. Since the size of the electric field applied to the liquid crystal molecules of a pixel depends on the voltage difference between the pixel electrode of the pixel and a common electrode, the display quality of the LCD device will be affected when the voltage level of the pixel electrode changes due to the impact from feed-through voltage VFD.
Typically, the impact of the feed-through voltage VFD can be eliminated or reduced by adjusting the voltage of the common electrode. However, due to the impact from the resistance and capacitance of a scan line, ΔVG will be reduced with an increase of the distance between pixels and the input end of the scan line (the input end is the end of the scan line connected to the scan line driving circuit 12, as depicted in FIG. 1). With increasing distance along the scan line from the input end, the increased resistance and capacitance causes the activation voltage on the scan line to be reduced, which in turn causes reduction of ΔVG.
In FIG. 1, ΔVG of the pixels P10A, P10B, and P10C will be such that (ΔVG)A>(ΔVG)B>(ΔVG)C, where (ΔVG)A represents ΔVG at pixel P10A, (ΔVG)B represents ΔVG at pixel P10B, and (ΔVG)C represents ΔVG at pixel P10C. Therefore, according to Eq. 1, when the CLC, CST, and CGD of the pixels are the same, ΔVFD of the pixels P10A, P10B, and P10C will be such that (VFD)A>(VFD)B>(VFD)C, where (VFD)A is the feed-through voltage for pixel P10A, (VFD)B is the feed-through voltage for pixel P10B, and so forth. Consequently, the impact from VFD cannot be eliminated by just adjusting the voltage level of the common electrode, which would cause a flicker problem in the picture of the liquid crystal display device.